Nuclear fuel elements



r 3,031,392 Patented Apr. 24, 1962 United States Patent Ofiiee Thisinvention. relates to improvements in nuclear fuel elements,particularly to fuel elements having improved corrosion resistant metaljackets. i

This application is a division of a copending application entitledAlloy. Especially Suited to Cladding Nuclear Fuel Elements, Serial No.687,606, filed'October 12, 1957, now. Patent No. 2,941,883.

Uranium dioxide and other nuclear fuel element materials by themselveslack .suificient structural strength to be v used in the form of rods.orthe like in nuclear reactors.

Powdered aluminum, stainless steel or other similar materials areoftencombined with'uranium. dioxideand other nuclear *fuel element materialsto facilitate shaping them- 1 into desired forms and to add structuralstrength, but

further support is necessary. 'Also, a barrier is required around thenuclear fuel element core materials at all times to protect the coreagainst corrosion, against erosion and abrasion and against the loss offission fragments. To satisfy these requirements fuel elements are oftenclad with a material having suitable mechanical and chemical properties.However, the known cladding materials are t not entirely "satisfactoryin service and consequently, a I search was made for a better clad,keeping in mind that such a material must have both structural strength,corrosion resistance and a low thermal neutron absorption cross-section.Materials having a high neutron absorption cross-section are not usefulbecause they lower the efliciency of the fuelfelement; In addition, thecladding material must have high strength at elevated temperatures andpossess sufficient ductility to be formable at reasonable workingtemperatures.

In accordance with this invention, the alloy to be used for cladding offuel elements consists essentially of about 3 to 6% weight percent'('i.e., w/o) chromium about 0.2 to 1 w/o titanium, about 4 to 11 w/oaluminum, about 0.5 to 4 w/o niobium, not more than about 0.15 w/ocarbon, and the balance iron.

More particularly, the above alloy may contain up to about 3 w/omolybdenum and up to about 4.5 w/o silicon.

In a more particular aspect of this invention, an article i ofmanufacture is proposed which consists essentially of a nuclear fuelelement which has the outer surface thereof clad with the alloymentioned above. The cladding material of this invention is ofrelatively low thermal neutron absorption cross-section and for thatreason it is especially suitedfor use in nuclear reactors.

In the initial stages of finding a material having the desired corrosionresistance, strength, ductility, formability and low neutron absorbingcross-section to clad fuel elements, various alloys were tried butunfortunately it was found that while they possessed one or two of thesedesired properties, they did not possess all of them. On

the other hand, some alloys had a suitable neutron absorptioncross-section but didjnot possess satisfactory mechanical or chemicalproperties. Among the various alloys which were tried, the aluminum-ironalloys showed ing to the alloy a low thermal neutron absorptioncrosssection and high corrosion resistance. However, when the quantityof aluminum is increased to a certain level in the alloy, the mechanicalproperties and fabricability are undesirably changed and for this reasonit was at first thought that an aluminum containing alloy was not ofpractical importance. Later it was found that small amounts of niobiumalloyed with chromium, aluminum and iron, resulted in an alloy havingexcellent mechancombination with titanium, cooperates to an unusualextent in minimizing any, adverse effects from carbon, and further,these elements jointly give a greater resistance to corrosion that canbe expected from the individual effects of each element. an amount ofabout.0.20 to 1% by weight based on the total composition. Excellentresults are obtained by the use of 0.5 W/o of titanium. Silicon andmolybdenum can also be added to the alloy in order to enhance itsmechanical properties. Molybdenum is added in an amount of about 0.5 to3 w/o based on the total weight of the composition. Silicon, in additionto enhancing the mechanical properties, serves to'reduce the tendency ofthe alloy to creep or scale at high temperature. Silicon is added in anamount of about O.1.to 4.5 w/o, based on the total weight of thecomposition. Silicon and molybdenum are optional alloy ingredients andare used only when it is desired to obtain further improvements incertain properties of the'alloy.

This alloy can be manufactured by blending the desired percentages ofthe above ingredients while in solid form, vacuum orair melting theresultant alloy, casting the melt into the desired shape, employingstandard melting and casting procedures. This cast alloy can then berolled and cut to form in the desired clad thicknesses and shapesdesired.

It was found that the concentration of aluminum should be carefullycontrolled in order to produce an promise, because aluminum has theproperty .ofimpartalloy having the desired low thermal neutronabsorption cross-section while also having desired mechanicalproperties. It was found, after careful experimentation, that thealuminum content of the alloy is most effective at about 4 to 11% byweight, based on the total composition. At this concentration ofaluminum, niobium is controlled within the range of 0.5 to 4% by weightbased on the total composition. Similarly, the chromium content of thealloy is maintained at about 3 to 6% by weight. A higher concentrationof chromium is not desired because it undesirably increases the neutronabsorption cross-section of the alloy while imparting little addedimprovements in the alloy properties. It is believed that a higherconcentration of chromium inhibits the forma tion of the iron-aluminumsuper lattice with attendant poor mechanical properties such as lowerductility and fabricability. The balance of the alloy is iron. However,it is also important to maintain the carbon at a level not greater than0.15% by weight. Higher con centrations of carbon have an adverse effecton the mechanical properties of the alloy. It is apparent that Titaniumis added. to alloy in the concentrations of the alloying ingredients arecontrolled to provide an alloy having low thermal neutron absorptioncross-section, excellent ductility, low corrosion susceptibility atelevated temperatures, high strength at elevated temperatures andexcellent bonding properties for adherence to the fuel core. In respectto the bonding properties of the alloy of this invention, by methodsused in fuel element fabrication, it Was found that it is comparable tostainless steel. At present it is known that stainless steel possessesexcellent mechanical properties for use in cladding fuel elements but ithas an undesirably high thermal neutron absorption cross-section and isa source of difiiculty in reprocessing spent fuel elements by chemicaldissolution techniques.

In order to evaluate the alloy of the present invention, a comparisonwas made with other alloys that might be used as cladding materials. Thealloys used in thi comparison are described below, the alloy of thepresent invention being Example 1.

Example I An alloy containing 6% chromium, 8% aluminum, 2% niobium, 0.5%titanium, and the balance iron.

Example 2 16% aluminum, 3.3% molybdenum, and the balance iron.

Example 3 Nickel-chromium, 18-8 stainless steel. In Table I below, themechanical properties at room temperature and an elevated temperatureare given for Examples 1 and 2.

TABLE I invention, i.e., Example 1, is a great deal more ductile thanthe alloy containing a high content of aluminum. Furthermore, thestrength of the alloy of this invention compares favorably with that ofExample 2.

The thermal neutron absorption cross-section of Example 1 was comparedwith that of Example 3. As previously indicated, stainless steelpossesses excellent mechanical properties but the material has anundesirable neutron absorption cross-section. Table 11 below illustratesthis point.

TABLE II Macroscopic Thermal Neutron Capture Crosssection RelativeCross-section,

Ex. N0. 3=100 Material The fabricability of Examples 1 and 2 werecompared and the results of this comparison are given in Table IIIbelow.

TABLE IIL-FABRIOABILITY Tensile Test Minimum Material (R.T.) BendMaehine- Hot- Form- Elonga- Radius, ability Bonding ability tion, in.percent Ex. No.1 14.0 1T Good... Excellent.. Fair. Ex. No.2 1.0 24'?Poor.... Poor Poor.

it can be seen from Tables I, II and III above that the alloy of thisinvention has excellent mechanical and excellent thermal neutronabsorption cross-section properties. This new alloy actually possessesthe desirable mechanical properties of stainless steel and the desirablelow thermal neutron absorption cross-section of high aluminum containingalloys. I

The alloy of the present invention also has excellent corrosionresistance properties in the presence of air,

Water, nitrogen or carbon dioxide at high temperatures. This resistanceis better than that of any of the other alloys set forth in the aboveexamples and is generally better than that of stainless steel oraluminum.

In commercial practice, the clad fuel element is used for a period ofabout 18 months. The clad has a thickness of about 0.005 to about 0.030inch. It has been found that fuel elements clad with the alloy of thisinvention can be reprocessed much easier than fuel elements containingstainless steel as the cladding material. .This is an importantadvantage from a commercial standpoint.

The alloy of this invention can also be used to clad other elements of anuclear reactor such as the control rods, etc., and it is contemplatedthat it can be used in numerous other applications where its uniquecharacteristics are desired.

Having thus provided a Written description of our invention along withspecific examples thereof, it should be understood that no unduerestrictions or limitations are to be imposed by reason thereof, butthat the present invention is defined by the appended claims.

We claim:

1. An article of manufacture consisting essentially of a fuel elementcontaining nuclear fuel material and having the outer surface thereofclad with an alloy consisting by weight essentially of about 3 to 6%chromium, about 4 to 11% aluminum, about 0.5 to 4% niobium, not morethan about 0.15% carbon, about 0.2 to 1% titanium, and the balance iron.

2. An article of manufacture consisting essentially of a nuclear fuelelement containing fissionable material and having the outer surfacethereof clad with an alloy consisting by weight essentially of about 3to 6% chromium, about 4 to 11% aluminum, about 0.5 to 4% niobium, notmore than about 0.15% carbon, about 0.2 to 1% titanium, up to about 4.5%silicon, up to about 3% molybdenum and the balance iron.

3. An article of manufacture consisting essentially of a fuel elementcontaining fissionable material and having the outer surface thereofclad with an alloy consisting essentially by weight of about 3% t0 6%chromium, about 4% to 1.1% aluminum, about 0.5% to 4% niobium, not morethan about 0.15% carbon, about 0.2% to 1% titanium, at least onematerial selected from the group consisting of silicon and molybdenum,said silicon and molybdenum being present in said alloy in amounts notgreater than about 4.5% and 3%, respectively, balance lron.

No references cited.

1. AN ARTICLE OF MANUFACTURE CONSISTING ESSENTIALLY OF A FUEL ELEMENTCONTAINING NUCLEAR FUEL MATERIAL AND HAVING THE OUTER SURFACE THEREOFCLAD WITH AN ALLOY CONSISTING BY WEIGHT ESSENTIALLY OF ABOUT 3 TO 6%CHROMIUM, ABOUT 4 TO 11% ALUMINUM, ABOUT 0.5 TO 4% NIOBIUM, NOT MORETHAN ABOUT 0.15% CARBON, ABOUT 0.2 TO 1% TITANIUM, AND THE BALANCE IRON.